Austenitic stainless steels excellent in flexibility

ABSTRACT

Austenitic stainless steels excellent in flexibility are provided. The austenitic stainless steel excellent in flexibility includes, by weight percent, 0.1 to 0.65% of Si, 1.0 to 3.0% of Mn, 6.5 to 10.0% of Ni, 16.5 to 18.5% of Cr, 6.0% or less of Cu (excluding 0), 0.13% or less of (C+N) (excluding 0), and the remainder including Fe and unavoidable impurities, wherein the work hardening formula H1 defined by the following formula is 300 or less. 
       H1=−459+79.8Si−10.2Mn−8.16Ni+48.0Cr−13.2Cu+623(C+N).

TECHNICAL FIELD

The present invention relates to austenitic stainless steels excellentin flexibility.

BACKGROUND ART

Attempts have been made to apply stainless steel to air conditionerrefrigerant piping for conventional household use and automobiles. Thisis because it is not only excellent in corrosion resistance but alsorelatively low in material cost.

However, work such as bending of piping is essential since installationof air conditioner refrigerant piping is limited by the installationspace, but there exists a problem in that the general stainless steeldoes not have the flexibility that must be provided in pipinginstallation.

A metal material has a property that when subjected to strain such astensile or compression, work hardening occurs and it becomes stronger asit is subjected to strain. The bending of pipe is a complex action oftension and compression, and as the degree of bending increases, thematerial becomes more hardened. In particular, SUS 304, which is mostwidely used as austenitic stainless steel, has a severe degree of workhardening, and it is very difficult to bend piping by manpower in aspace where air conditioner piping work is required.

Work hardening is expressed as TS-YS, which is the difference betweenthe yield strength (YS) indicating the strength at the start of materialdeformation and the tensile strength (TS) indicating the maximumstrength due to maximization of work hardening of the material. In otherwords, in order to bend the material easily with manpower, a material inwhich TS-YS is minimized by suppressing such work hardening phenomenonis required.

In the austenitic stainless steels, Cr, Ni, Mn, Cu, C and N elements aremainly added. Although many steel types have been produced by varyingthe content of these elements, an optimum component control method forexcellent flexibility has not been disclosed. In the present invention,it was attempted to produce materials having excellent flexibility byminimizing work hardening through control of these elements.

It should be understood that the foregoing description of the backgroundart is merely for the purpose of promoting an understanding of thebackground of the present invention, and is not to be construed asadmission that it is the prior art known to those skilled in the art.

(Patent Literature 0001) KR 10-2010-0099726 A (2010.09.13)

DISCLOSURE OF INVENTION Technical Problem

An object of the present invention is to provide austenitic stainlesssteels excellent in flexibility by controlling the content of componentelements affecting the degree of work hardening and controlling the sizeof crystal grains in order to solve such conventional problems.

Technical Solution

To achieve the object described above, an austenitic stainless steelexcellent in flexibility according to the present invention ischaracterized by comprising, by weight percent, 0.1 to 0.65% of Si, 1.0to 3.0% of Mn, 6.5 to 10.0% of Ni, 16.5 to 18.5% of Cr, 6.0% or less ofCu (excluding 0), 0.13% or less of (C+N) (excluding 0), and theremainder comprising Fe and unavoidable impurities, wherein the workhardening formula H1 defined by the following formula is 300 or less.

H1=−459+79.8Si−10.2Mn−8.16Ni+48.0Cr−13.2Cu+623(C+N)

The austenitic stainless steel excellent in flexibility according to thepresent invention is characterized by having the size of structure (D)of 20 to 40 μm.

To achieve the object described above, an austenitic stainless steelexcellent in flexibility according to the present invention ischaracterized by comprising, by weight percent, 0.1 to 0.65% of Si, 1.0to 3.0% of Mn, 6.5 to 10.0% of Ni, 16.5 to 18.5% of Cr, 6.0% or less ofCu (excluding 0), 0.13% or less of (C+N) (excluding 0), and theremainder comprising Fe and unavoidable impurities, wherein the workhardening formula H2 defined by the following formula is 300 or less.

H2=4.27+0.875(−459+79.8Si−10.2Mn−8.16Ni+48.0Cr−13.2Cu+623(C+N))−287D (D:the size of structure)

The size of structure (D) is characterized by being 20 to 300 μm.

An austenitic stainless steel excellent in flexibility according to thepresent invention is characterized by comprising, by weight percent, 0.1to 0.65% of Si, 1.0 to 3.0% of Mn, 6.5 to 10.0% of Ni, 16.5 to 18.5% ofCr, 6.0% or less of Cu (excluding 0), 0.13% or less of (C+N) (excluding0), and the remainder comprising Fe and unavoidable impurities, whereinM_(d30) defined by the following formula is 0 or less.

M_(d30)=551−462(C+N)−9.2Si−8.1Mn−29(Ni+Cu)−13.7Cr

It is preferable that M_(d30) is −100 to 0.

The difference value between TS (tensile strength) and YS (yieldstrength) is characterized by being 300 MPa or less.

Advantageous Effects

The present invention has an advantage that austenitic stainless steelsexcellent in flexibility can be produced by controlling the content ofelements, the size of crystal grains, and the like.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing a correlation between the work hardeningformula H1 and actually measured values of work hardening degree;

FIG. 2 is a diagram showing a change of the work hardening formula H1according to the size of crystal grains:

FIGS. 3 to 5 show size distributions of crystal grains:

FIG. 6 is a diagram showing a correlation between the modified workhardening formula H2 and actually measured values of the work hardeningdegree, and

FIG. 7 is a diagram showing a correlation between the austenitestabilization index and actually measured values of the work hardeningdegree.

MODE FOR INVENTION

Hereinafter, austenitic stainless steels excellent in flexibilityaccording to preferred embodiments of the present invention will bedescribed with reference to the accompanying drawings.

An austenitic stainless steel according to the present invention ischaracterized by containing, by weight percent, 0.1 to 0.65% of Si, 1.0to 3.0% of Mn, 6.5 to 10.0% of Ni, 16.5 to 18.5% of Cr, 6.0% or less ofCu (excluding 0), 0.13% or less of (C+N) (excluding 0), and theremainder comprising Fe and unavoidable impurities.

The reasons for limiting the numerical values of the componentsconstituting the austenitic stainless steel excellent in flexibility ofthe present invention will be described below.

C+N should be added to 0.13 wt % or less.

C and N not only harden the austenitic stainless steel as interstitialsolid solution strengthening elements but also increase the workhardening degree of the material by hardening the strain inducedmartensite generated during processing if the contents of C and N arehigh. Therefore, there is a need to limit the content of C and N, and inthe present invention, the content of C+N is limited to 0.13% or less.

Si is added in a controlled amount with the range of 0.1 to 0.65 wt %.

Since Si is an element added essentially for deoxidation, 0.1% or moreis added.

However, when an excessively high content of Si is added, the materialis hardened and the corrosion resistance is lowered by forminginclusions in association with oxygen, so the upper limit is limited to0.65%.

Mn is added in a controlled amount with the range of 1.0 to 3.0 wt %.

Mn, which is an element not only added essentially for deoxidation butalso increases the degree of stabilization of the austenite phase, isadded at 1.0% or more for maintaining the austenite balance. However,the addition of an excessively high content of Mn reduces the corrosionresistance of the material, so the upper limit is limited to 3.0%.

Ni is added in a controlled amount with the range of 6.5 to 10.0 wt %.

Ni is not only effective for improving the corrosion resistance such aspitting corrosion resistance by being added with Cr in combination, butalso can increase softening of austenite steel when its content isincreased.

In addition, Ni is an element contributing to improvement of phasestability of austenitic stainless steel, and is added at 6.5% or more inorder to maintain an austenite balance. However, the addition of anexcessively high content of Ni results in an increase in the cost of thesteel, so the upper limit is limited to 10.0%.

Cr is added in a controlled amount with the range of 16.5 to 18.5 wt %.

Cr is an indispensable element for improving the corrosion resistance,and in order to be used for general purpose, 16.5% or more of Cr shouldbe added. However, the addition of an excessively high content of Crcauses austenite phase hardening and increases the cost, so the upperlimit is limited to 18.5%.

Cu is added in a controlled amount with the range of 6.0 wt % or less.

Cu can cause softening of the austenite steel. However, the addition ofan excessively high content of Cu lowers the hot workability and canrather harden the austenite phase, so the upper limit is limited to6.0%.

In order to attain the object of the present invention, the componentcontrol method provided by the present invention is important. In orderto express this specifically, the following description will be madewith reference to the embodiments of the present invention. Thematerials described in the following embodiments were prepared bypreparing ingots with a 150 mm thickness, heating them to 1,250° C., hotrolling them to 3 mm, and then heat treating them at 1,100° C. for 60seconds or more. However, such a manufacturing method does not limit thecharacteristics of the material provided in the present invention, butmerely adopts one of the conventional methods of manufacturingaustenitic stainless steel, and is merely an example of producing amaterial for evaluating characteristics. The characteristics of thematerial change depending on the component control method provided bythe present invention. The yield strength YS and the tensile strength TSare values obtained by uniaxially tensioning the material.

TABLE 1 Classification Si Mn Ni Cr Cu C + N TS-YS H1 Invention 0.4 2.78.0 17.3 2.7 0.019 281 292 Example 1 Invention 0.4 1.7 9.6 17.4 3.20.028 277 284 Example 2 Invention 0.4 1.7 9.6 17.4 3.2 0.024 273 281Example 3 Invention 0.4 2.8 9.6 17.5 3.1 0.010 276 271 Example 4Invention 0.4 2.7 9.6 17.4 3.2 0.011 279 267 Example 5 Invention 0.4 2.79.7 17.5 3.2 0.019 277 273 Example 6 Invention 0.4 2.7 9.6 17.4 3.20.041 280 285 Example 7 Invention 0.4 1.2 8.3 16.9 2.1 0.016 287 286Example 8 Invention 0.4 1.2 8.4 16.9 2.2 0.033 295 294 Example 9Invention 0.4 1.2 8.1 17.0 2.8 0.018 288 284 Example 10 Invention 0.41.2 8.0 17.0 2.7 0.036 293 295 Example 11 Invention 0.4 1.2 8.4 16.8 2.70.017 280 275 Example 12 Invention 0.4 1.2 8.4 17.0 2.7 0.036 287 293Example 13 Invention 0.6 1.2 7.6 16.9 3.0 0.017 283 296 Example 14Invention 0.6 1.2 7.6 16.9 4.0 0.021 286 286 Example 15 Invention 0.61.2 7.6 16.7 5.0 0.020 274 263 Example 16 Comparative 0.6 1.2 7.6 16.92.1 0.056 328 329 Example 1 Comparative 0.4 1.0 7.9 17.7 0.2 0.088 407399 Example 2 Comparative 0.6 1.2 7.5 16.8 2.0 0.021 309 308 Example 3

H1 shown in Table 1 is defined by the following formula.

H1=−459+79.8Si−10.2Mn−8.16Ni+48.0Cr−13.2Cu+623(C+N)

In the present invention, in order to obtain an austenitic stainlesssteel excellent in flexibility by controlling the TS-YS value to 300 MPaor less, the H1 values are defined using the component elementsconstituting the present invention, and the correlation between the H1values and the actually measured TS-YS values were analyzed.

As shown in FIG. 1, it can be seen that the relationship between the H1values obtained through the component control and the actually measuredTS-YS values is shown, and the above description is implemented. Inparticular, as shown by a dotted line, a linearly smooth relationship isestablished therebetween. Therefore, it can be seen that even if thelower limit of the H1 value is not set in the present invention, it ispossible to manufacture an austenitic steel having more excellentflexibility through production of a material having a lower H1 value.

On the other hand, the crystal grain size of the austenitic stainlesssteel produced by a conventional manufacturing process is generally30±10 μm.

As shown in Table 2, the crystal grain size (D) of the austeniticstainless steel excellent in flexibility of the present invention isalso present in the interval of 30±10 μm, and it can be seen that whenH1 is obtained as 329 as in Comparative Example 1 of Table 2, the actualTS-YS value is obtained as 328, indicating that the flexibility is notgood.

As above, it can be seen that the values of H1 and the actual TS-YSvalues have similar values at crystal grain sizes of the range of 30 f10 μm, which is also confirmed through FIG. 2.

However, in a case when the size of the crystal grains exceeds the rangeof 30±10 μm, it can be seen that the actual TS-YS values are less than300 MPa even if the values of H1 exceed 300 MPa, which is also confirmedthrough Invention Examples 17, 18, 19, 20 and 21 in Table 2 and thesection marked as ellipse in FIG. 2.

If the crystal grain size is large, surface irregularity defect calledorange peel occurs during processing. However, if the smoothness of thesurface is not important or can be corrected through polishing and canbe ignored, even if the crystal grain size is large, it is not a bigproblem.

FIGS. 3 to 5 show size distributions of crystal grains, in which FIG. 3is a structure photograph showing the crystal grain size of theaustenitic stainless steel according to the following Invention Example6, FIG. 4 is a structure photograph showing the crystal grain size ofthe austenitic stainless steel according to the following ComparativeExample 6, and FIG. 5 is a structure photograph showing the crystalgrain size of the austenitic stainless steel according to the followingInvention Example 17.

In the present invention, a modified work hardening formula H2 isprovided so as to obtain a material having a low work hardening degreeeven when the crystal grain size is larger than usual.

H2=4.27+0.875H1−0.287D

As shown in Table 2 and FIG. 6, it can be seen that austenitic stainlesssteels excellent in flexibility can be produced by controlling the rangeof the modified work hardening formula H2 to 300 MPa or less.

TABLE 2 TS-YS H1 D H2 Invention 281 292 29 289 Example 1 Invention 277284 31 282 Example 2 Invention 273 281 33 279 Example 3 Invention 276271 29 271 Example 4 Invention 279 167 31 268 Example 5 Invention 277173 32 272 Example 6 Invention 280 285 35 282 Example 7 Invention 269336 223 273 Example 17 Invention 247 316 218 256 Example 18 Invention240 301 209 246 Example 19 Invention 267 333 284 253 Example 20Invention 283 316 93 292 Example 21 Comparative 328 329 33 321 Example 1Comparative 337 406 210 337 Example 4 Comparative 371 406 990 372Example 5 Comparative 313 336 72 316 Example 6

Table 3 shows the component contents of Invention Examples 17 to 21 andComparative Examples 4 to 6 disclosed in Table 2.

TABLE 3 Classification Si Mn Ni Cr Cu C + N Invention 0.6 1.2 7.5 16.73.9 0.119 Example 17 Invention 0.6 1.3 7.6 17.0 5.0 0.087 Example 18Invention 0.6 1.3 7.9 17.1 5.8 0.075 Example 19 Invention 0.5 1.1 6.917.1 4.4 0.091 Example 20 Invention 0.6 1.3 7.6 17.0 5.0 0.087 Example21 Comparative 0.2 1.4 8.1 18.1 0.2 0.105 Example 4 Comparative 0.2 1.48.1 18.1 0.2 0.105 Example 5 Comparative 0.6 1.2 7.5 16.7 3.9 0.119Example 6

On the other hand, the TS-YS values may be limited by the followingaustenite stability M_(d30).

As shown in FIG. 7, it can be seen that when M_(d30) exceeds 0, theTS-YS values greatly increase, and in the range where M_(d30) is 0 orless, the TS-YS values do not react sensitively to M_(d30) but remain ata constant low level.

In order to maintain the M_(d30) in the range of 0 or less, Si, Mn, Ni,Cu and Cr which are the main additive elements must be added. In thepresent invention, M_(d30)-related component parameters for maintainingthe TS-YS values at 300 MPa or less are presented.

TABLE 4 TS-YS M_(d30) Invention Example 1 281 −30 Invention Example 2227 88 Invention Example 3 273 85 Invention Example 4 276 88 InventionExample 5 279 88 Invention Example 6 277 −97 Invention Example 7 280−102 Invention Example 8 287 −2 Invention Example 9 295 −14 InventionExample 10 288 −18 Invention Example 11 293 −22 Invention Example 12 280−21 Invention Example 13 287 −34 Invention Example 14 283 −13 InventionExample 15 286 −41 Invention Example 16 274 −69 Comparative Example 1328 −1 Comparative Example 2 407 20 Comparative Example 3 309 20

As shown in Table 4, when the values are maintained at 0 or less, theTS-YS values can be maintained at 300 MPa or less, which indicates thatthe flexibility is improved.

On the other hand, in order to lower the M_(d30) values, the componentelement contents should be further increased. In order to reduce thecost, the lower limit value is preferably limited to −100.

While the present invention has been particularly shown and describedwith reference to specific embodiments thereof, it will be understood bythose skilled in the art that the present invention may be variouslymodified and changed without departing from the technical idea of thepresent invention provided by the following claims.

INDUSTRIAL APPLICABILITY

The austenitic stainless steels excellent in flexibility according tothe embodiments of the present invention are applicable to airconditioner refrigerant piping and the like for domestic use andautomobiles.

1. An austenitic stainless steel excellent in flexibility beingcharacterized by comprising: by weight percent, 0.1 to 0.65% of Si, 1.0to 3.0% of Mn, 6.5 to 10.0% of Ni, 16.5 to 18.5% of Cr, 6.0% or less ofCu (excluding 0), 0.13% or less of (C+N) (excluding 0), and theremainder comprising Fe and unavoidable impurities, wherein the workhardening formula H1 defined by the following formula is 300 or less.H1=−459+79.8Si−10.2Mn−8.16Ni+48.0Cr−13.2Cu+623(C+N)
 2. The austeniticstainless steel excellent in flexibility according to claim 1, beingcharacterized by having the size of structure (D) of 20 to 40 μm.
 3. Anaustenitic stainless steel excellent in flexibility being characterizedby comprising: by weight percent, 0.1 to 0.65% of Si, 1.0 to 3.0% of Mn,6.5 to 10.0% of Ni, 16.5 to 18.5% of Cr, 6.0% or less of Cu (excluding0), 0.13% or less of (C+N) (excluding 0), and the remainder comprisingFe and unavoidable impurities, wherein the work hardening formula H2defined by the following formula is 300 or less.H2=4.27+0.875(−459+79.8Si−10.2Mn−8.16Ni+48.0Cr−13.2Cu+623(C+N))−287D (D:the size of structure)
 4. The austenitic stainless steel excellent inflexibility according to claim 3, being characterized by having the sizeof structure (D) of 20 to 300 μm.
 5. An austenitic stainless steelexcellent in flexibility being characterized by comprising: by weightpercent, 0.1 to 0.65% of Si, 1.0 to 3.0% of Mn, 6.5 to 10.0% of Ni, 16.5to 18.5% of Cr, 6.0% or less of Cu (excluding 0), 0.13% or less of (C+N)(excluding 0), and the remainder comprising Fe and unavoidableimpurities, wherein M_(d30) defined by the following formula is 0 orless.M_(d30)=551−462(C+N)−9.2Si−8.1Mn−29(Ni+Cu)−13.7Cr
 6. The austeniticstainless steel excellent in flexibility according to claim 5, whereinM_(d30) is −100 to
 0. 7. The austenitic stainless steel excellent inflexibility according to claim 1, wherein the difference value betweenTS (tensile strength) and YS (yield strength) is 300 MPa or less.
 8. Theaustenitic stainless steel excellent in flexibility according to claim2, wherein the difference value between TS (tensile strength) and YS(yield strength) is 300 MPa or less.
 9. The austenitic stainless steelexcellent in flexibility according to claim 3, wherein the differencevalue between TS (tensile strength) and YS (yield strength) is 300 MPaor less.
 10. The austenitic stainless steel excellent in flexibilityaccording to claim 4, wherein the difference value between TS (tensilestrength) and YS (yield strength) is 300 MPa or less.
 11. The austeniticstainless steel excellent in flexibility according to claim 5, whereinthe difference value between TS (tensile strength) and YS (yieldstrength) is 300 MPa or less.
 12. The austenitic stainless steelexcellent in flexibility according to claim 6, wherein the differencevalue between TS (tensile strength) and YS (yield strength) is 300 MPaor less.